There are a number of devices which incorporate transducer type probes plugged into a monitor device which processes the signal from the transducer and provides a readout understandable by the user: air flow monitors, ultrasound fetal monitors, EKG monitors, thermal anemometers, fluid flow velocity measurement systems, hygrometry probes, and the like, Such devices generally measure physical parameters such as temperature, fluid flow velocity, and the like.
One problem with these types of devices is that they must be calibrated before use because of manufacturing differences between one transducer and another. For example, a thermistor in one probe may indicate a voltage V.sub.1 when subjected to a temperature T.sub.1 while another thermistor in another probe may indicate a voltage V.sub.2 when subjected to the same temperature T.sub.1.
So, the probes are calibrated prior to use. One manufacturer of an air flow measurement system provides the user with a diskette containing all the calibration coefficients for each probe manufactured by serial number. The user connects one or more probes to the monitor, enters the serial numbers and location of those probes, and then a routine automatically calibrates the monitor.
Another older method is to subject the individual probes to known conditions, read the signal provided by the probe, and adjust the monitor until it displays the "correct" reading. See U.S. Pat. No. 4,537,068, column 13, line 64 through column 14, line 6. Some devices even come with their own calibration equipment. See, for example, the "calibrator" described in conjunction with the fluid (blood) flow velocity measuring device described in U.S. Pat. No. 3,595,079.
The problem with these prior art methods is that the individual probes must be calibrated before they are used and, although trained engineers may be able to accomplish this task fairly accurately, technicians and other users may fail to calibrate the device or fail to calibrate the device correctly. Worse, in units with many probes and/or interchangeable probes, the disadvantage lies in the time involved in calibrating the unit each time a probe is added, removed, or replaced. For example, in the case of the air flow measurement system described above, each time a new probe is used, or probes are interchanged or moved, the calibration diskette must be loaded and the calibration routine run again. In the case of the blood flow velocity measuring device of U.S. Pat. No. 3,595,079, the "calibrator" must be used by trained personnel to calibrate the device each time a new probe is used on a new patient by a physician.
Moreover, in addition to the flow rate, there are a number of implementations where it is desirable to know the flow direction. For example, in clean rooms, systems must be used to make sure there is a positive air flow out of each clean room. For hospital isolation rooms, there is a need to ensure that there is a positive air flow into the isolation room. Unfortunately, most prior art air flow measurement systems fail to provide the user with an indication relating to the direction of air flow.